Abelacimab
Medium EvidenceAbelacimab (development code MAA868) is an investigational fully human monoclonal antibody being developed as a new kind of blood thinner (anticoagulant). It works by targeting Factor XI, a clotting protein that sits high in the coagulation 'cascade.' Abelacimab is described as a dual inhibitor: it binds the catalytic domain of Factor XI and locks the protein in its inactive (zymogen) shape, so it blocks both Factor XI itself and its activated form, Factor XIa, preventing the enzyme from being switched on by upstream triggers such as Factor XIIa or thrombin. The appeal of the Factor XI target is a long-standing observation in human biology: Factor XI contributes strongly to pathological clot formation (thrombosis) but only modestly to normal wound-sealing (hemostasis), so blocking it may prevent strokes and clots while causing much less bleeding than standard anticoagulants. Abelacimab is given as a once-monthly subcutaneous injection (with an intravenous loading option in some settings) and has a long duration of action. Its lead uses are stroke prevention in atrial fibrillation and the treatment and prevention of cancer-associated blood clots (venous thromboembolism, VTE). In the Phase 2b AZALEA-TIMI 71 trial, abelacimab reduced bleeding dramatically compared with the direct oral anticoagulant rivaroxaban - so much so that the study was stopped early - and it is now in Phase 3 development. Abelacimab was developed by Anthos Therapeutics, which Novartis originally helped launch and then reacquired in 2025. It is an investigational prescription biologic administered under medical supervision; it is not approved, and it is not a supplement, nootropic or research chemical.
What It Is
Abelacimab (MAA868) is a fully human IgG1 monoclonal antibody that inhibits Factor XI, one of the clotting factors in the intrinsic (contact) pathway of coagulation. It is often called a 'dual' Factor XI inhibitor because it binds a specific region (the catalytic domain) of the Factor XI zymogen and holds the molecule in its inactive conformation, which both prevents Factor XI from being activated and neutralizes any Factor XIa that forms; upstream activators such as Factor XIIa and thrombin can no longer switch the enzyme on. The scientific rationale for targeting Factor XI is a decades-old human genetics observation: people with inherited Factor XI deficiency are protected from venous and some arterial thrombosis yet have only a mild bleeding tendency, and Factor XII deficiency causes essentially no bleeding at all. This suggests that the intrinsic pathway is more important for pathological clotting than for the normal hemostasis that seals wounds, so blocking Factor XI could 'uncouple' thrombosis prevention from bleeding risk - the central promise of the Factor XI class and the reason it is described as a potential 'hemostasis-sparing' anticoagulant. Abelacimab is administered subcutaneously once a month (with an intravenous loading dose used in some regimens) and has a long half-life on the order of a month, giving durable, steady Factor XI suppression from a single monthly injection. Its most advanced application is stroke prevention in atrial fibrillation (AFib), where clots that form in the heart can travel to the brain. In the Phase 2b AZALEA-TIMI 71 trial (NCT04755283), 1,287 patients with AFib and moderate-to-high stroke risk were randomized to abelacimab (150 mg or 90 mg subcutaneously once monthly) or to the standard direct oral anticoagulant rivaroxaban (20 mg orally once daily). In September 2023 the independent data-monitoring committee halted the trial early because of an 'overwhelming' reduction in bleeding with abelacimab: at the 150 mg dose, major or clinically relevant non-major bleeding fell by roughly 67%, major bleeding by about 74%, and major gastrointestinal bleeding by about 93% versus rivaroxaban, with the results later published in the New England Journal of Medicine. Because AZALEA was a bleeding-focused Phase 2 study, it was not powered to prove stroke prevention, so whether abelacimab prevents strokes as effectively as existing anticoagulants is the key question its Phase 3 trials are designed to answer. That Phase 3 program includes LILAC-TIMI 76 (NCT05712200), which is testing abelacimab against placebo in roughly 1,900 AFib patients who are considered unsuitable for, or unable to tolerate, conventional oral anticoagulation - a population that currently often goes untreated - with a primary endpoint of ischemic stroke or systemic embolism and data anticipated in 2026. A second Phase 3 effort targets cancer-associated thrombosis (VTE), where cancer patients face both high clotting risk and high bleeding risk on standard therapy: the ASTER study (NCT05171049) compares abelacimab with the oral anticoagulant apixaban, and the MAGNOLIA study (NCT05171075) compares it with the low-molecular-weight heparin dalteparin in gastrointestinal/genitourinary cancers, together enrolling roughly 2,700 patients; abelacimab received FDA Fast Track designation in this setting, and readouts are expected in 2026. Abelacimab was developed by Anthos Therapeutics, a company launched in 2019 by Blackstone Life Sciences and Novartis; Novartis reacquired Anthos in a deal that closed on April 3, 2025 (about $925 million upfront and up to roughly $3.1 billion with milestones), bringing abelacimab back into Novartis's late-stage cardiovascular pipeline. Abelacimab belongs to a broader wave of Factor XI-directed anticoagulants that also includes the small-molecule Factor XIa inhibitors asundexian and milvexian and the antisense agent fesomersen; the class as a whole is being watched as a possible next generation of safer blood thinners. Abelacimab is investigational and not approved by any regulator; it is a prescription biologic given under medical supervision and is not a consumer product.
Regulatory Status
Abelacimab (MAA868) is an investigational fully human monoclonal antibody that dually inhibits Factor XI and Factor XIa, and it is not approved by the FDA or any other regulatory agency. Its Phase 2b AZALEA-TIMI 71 trial (NCT04755283) in atrial fibrillation was stopped early in September 2023 for an overwhelming reduction in bleeding versus rivaroxaban, and results were published in the New England Journal of Medicine. Late-stage development is ongoing: LILAC-TIMI 76 (NCT05712200) is evaluating abelacimab versus placebo in roughly 1,900 AFib patients deemed unsuitable for oral anticoagulation (primary endpoint ischemic stroke or systemic embolism), and a cancer-associated VTE program - ASTER (NCT05171049, versus apixaban) and MAGNOLIA (NCT05171075, versus dalteparin), roughly 2,700 patients combined - holds FDA Fast Track designation, with readouts anticipated in 2026. Developed by Anthos Therapeutics, which was launched by Blackstone Life Sciences and Novartis in 2019; Novartis reacquired Anthos in a transaction that closed on April 3, 2025 (about USD 925 million upfront and up to roughly USD 3.1 billion with milestones). Administered by subcutaneous injection (with an intravenous loading option) in clinical settings only.
Why Researchers Study It
Researchers study abelacimab because it tests one of the most important ideas in modern cardiovascular medicine: that clotting and bleeding can be separated. Every widely used anticoagulant - from warfarin to the direct oral anticoagulants (DOACs) apixaban and rivaroxaban - lowers clot risk at the cost of raising bleeding risk, and fear of bleeding means many people who would benefit from anticoagulation, such as older or frail atrial-fibrillation patients, are never treated. Factor XI is attractive precisely because human genetics suggests it is far more important for pathological thrombosis than for the everyday hemostasis that stops wounds bleeding: inherited Factor XI deficiency protects against clots with only mild bleeding, and Factor XII deficiency causes clot protection with essentially no bleeding. Abelacimab lets scientists probe whether pharmacologically blocking Factor XI reproduces that 'hemostasis-sparing' profile in real patients. It is also a case study in antibody engineering and long-acting dosing: as a monoclonal antibody that locks Factor XI in its inactive zymogen state, abelacimab delivers deep, sustained Factor XI suppression from a single monthly subcutaneous injection, a very different pharmacology from daily oral pills. Its Phase 2 AZALEA-TIMI 71 result - a bleeding reduction so large the trial was stopped early - made it a reference point for the whole Factor XI field, and its Phase 3 trials (in anticoagulation-ineligible AFib and in cancer-associated thrombosis) are designed to answer the field's decisive question: can a Factor XI inhibitor prevent strokes and clots as well as it avoids bleeds? The answers are expected to shape how the next generation of anticoagulants is developed and used.
Proposed Mechanisms
- Factor XI targeting: abelacimab is a fully human monoclonal antibody that binds the catalytic domain of Factor XI, a clotting factor in the intrinsic (contact) pathway of the coagulation cascade.
- Dual XI/XIa inhibition via zymogen locking: by holding Factor XI in its inactive (zymogen) conformation, abelacimab both prevents the enzyme from being activated by upstream triggers (Factor XIIa or thrombin) and neutralizes its activated form, Factor XIa - hence its description as a dual inhibitor of Factor XI and Factor XIa.
- Hemostasis-sparing anticoagulation: because Factor XI contributes strongly to pathological clot amplification but only modestly to the hemostasis that seals wounds, blocking it is intended to reduce thrombosis (stroke, systemic embolism, venous thromboembolism) with substantially less bleeding than anticoagulants that act lower in the cascade.
- Long-acting, steady-state coverage: as an antibody with a half-life on the order of a month, abelacimab provides durable, consistent Factor XI suppression from a once-monthly subcutaneous injection (with an intravenous loading option), rather than the peaks and troughs of daily oral dosing.
- Maintenance anticoagulation, not acute rescue: abelacimab is intended for ongoing prevention or treatment of thrombosis; its long duration means the anticoagulant effect cannot be rapidly reversed, which is a key consideration around surgery, trauma and acute bleeding.
Evidence Snapshot
| Study Type | Model | Outcome | Link |
|---|---|---|---|
| Phase 2b randomized, active-controlled trial in atrial fibrillation (AZALEA-TIMI 71; NCT04755283), stopped early for benefit. | 1,287 adults with atrial fibrillation and moderate-to-high stroke risk randomized to abelacimab 150 mg or 90 mg subcutaneously once monthly versus rivaroxaban 20 mg orally once daily; median follow-up ~2.1 years; primary focus on major and clinically relevant non-major bleeding. | In September 2023 the independent data-monitoring committee halted the trial early because of an overwhelming reduction in bleeding with abelacimab. At the 150 mg dose, major or clinically relevant non-major bleeding was reduced by roughly 67%, major bleeding by about 74%, and major gastrointestinal bleeding by about 93% versus rivaroxaban, with results published in the New England Journal of Medicine. The study was not powered to establish stroke-prevention efficacy, which is being tested in Phase 3. | Source |
| Phase 3 placebo-controlled trial in anticoagulation-ineligible atrial fibrillation (LILAC-TIMI 76; NCT05712200), ongoing. | Approximately 1,900 patients with atrial fibrillation who are considered unsuitable for or unable to tolerate conventional oral anticoagulation, randomized to abelacimab 150 mg once monthly or placebo; primary endpoint is ischemic stroke or systemic embolism. | Designed to answer whether abelacimab prevents strokes and systemic emboli in a high-risk, currently often-untreated population while preserving its favorable bleeding profile. The trial is event-driven, with data anticipated in 2026; efficacy results are not yet available. | Source |
| Phase 3 program in cancer-associated venous thromboembolism (ASTER, NCT05171049; MAGNOLIA, NCT05171075), ongoing; FDA Fast Track designation. | Roughly 2,700 patients combined with cancer-associated VTE; ASTER compares abelacimab with the direct oral anticoagulant apixaban, and MAGNOLIA compares abelacimab with the low-molecular-weight heparin dalteparin in gastrointestinal/genitourinary cancers, using recurrent-VTE and bleeding endpoints. | Intended to test whether abelacimab can reduce clot recurrence with less bleeding than standard therapy in cancer patients, a group with simultaneously high clotting and bleeding risk. Readouts are expected in 2026; definitive efficacy and safety results are not yet reported. | Source |
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Safety & Cautions
- Abelacimab is an investigational anticoagulant (blood thinner) that is NOT approved by the FDA or any other regulator. It should only be received within a clinical trial or other supervised medical setting; any product sold as 'abelacimab' or 'MAA868' outside a legitimate clinical trial is unverified, unapproved and unsafe.
- As with all anticoagulants, the principal risk is bleeding. Although Factor XI inhibition is designed to cause less bleeding than standard anticoagulants - and abelacimab reduced bleeding markedly versus rivaroxaban in Phase 2 - bleeding can still occur, and there is no widely established specific reversal agent, which is relevant for emergencies, trauma or surgery.
- Abelacimab is a long-acting, once-monthly injection with an effect that persists for weeks. Its anticoagulant action cannot be quickly switched off, so timing around surgery, invasive procedures or acute bleeding requires specialist planning.
- The central unanswered question is efficacy: the pivotal AZALEA-TIMI 71 trial was designed around bleeding, not stroke prevention, so whether abelacimab prevents strokes and systemic clots as effectively as approved anticoagulants is still being tested in Phase 3 (LILAC-TIMI 76 and the cancer-VTE program). Until those results are in, its true benefit-risk balance is not established.
- As an injected biologic antibody, injection-site reactions and hypersensitivity reactions are possible, and its safety and efficacy in pregnancy, breastfeeding and children have not been defined.
- This is background information about an investigational medicine and its clinical evidence, not medical advice, and does not describe how any individual should be treated. Anticoagulation decisions must be made with a qualified clinician; people already taking blood thinners should never start, stop or change them on their own.
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Citations
- [1] Abelacimab versus Rivaroxaban in Patients with Atrial Fibrillation (AZALEA-TIMI 71) - New England Journal of Medicine PubMed
- [2] Safety and Tolerability of Abelacimab (MAA868) vs. Rivaroxaban in Patients With Atrial Fibrillation (AZALEA-TIMI 71; NCT04755283) - ClinicalTrials.gov PubMed
- [3] Study to Evaluate the Efficacy and Safety of Abelacimab in High-risk Patients With AF Deemed Unsuitable for Oral Anticoagulation (LILAC-TIMI 76; NCT05712200) - ClinicalTrials.gov PubMed
- [4] A Study Comparing Abelacimab to Apixaban in the Treatment of Cancer-Associated VTE (ASTER; NCT05171049) - ClinicalTrials.gov PubMed
- [5] A Study Comparing Abelacimab to Dalteparin in the Treatment of GI/GU Cancer and Associated VTE (MAGNOLIA; NCT05171075) - ClinicalTrials.gov PubMed
- [6] Pharmacokinetics and Pharmacodynamics of Abelacimab (MAA868), a Novel Dual Inhibitor of Factor XI and Factor XIa - Journal of Thrombosis and Haemostasis (ScienceDirect) PubMed
- [7] Novartis bolsters late-stage cardiovascular pipeline with agreement to acquire Anthos Therapeutics for USD 925 million upfront - Novartis PubMed
- [8] FDA Grants Fast Track Designation to Abelacimab for Thrombosis Associated With Cancer - OncLive PubMed
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